This invention relates to a one-way clutch, i.e. a clutch that transmits only torque in one direction from its input member to its output member.
There are a variety of one-way clutches, including those having a large torque capacity, which are used in automobiles, power tools, etc., and those having a smaller torque capacity, which are used e.g. in office machines such as copiers and PC's, and cameras.
Among such conventional one-way clutches, those of the type as disclosed in JP patent 2904660 are by far the most popular, irrespective of their torque capacity. This type of one-way clutch comprises inner and outer rings, and a plurality of engaging elements disposed between the inner and outer rings and retained by a retainer, also disposed between the inner and outer rings. Resilient members bias the engaging elements in one circumferential direction to keep the engaging elements in engagement with both of the inner and outer rings when the inner and outer rings rotate relative to each other in one direction. The engaging elements are typically rollers or sprags.
JP patent publication 5-332409 discloses another type of one-way clutch, which comprises a cylindrical member having a cylindrical inner surface, a shaft inserted in the cylindrical member and having a plurality of cam surfaces on its outer periphery, opposite the cylindrical inner surface of the cylindrical member, and friction members having inner and outer arcuate surfaces and each received in a wedge-shaped space defined between the cylindrical inner surface of the cylindrical member and one of the cam surfaces. Each friction member is biased toward the narrower end of the wedge-shaped space by a resilient member so that when the shaft and the cylindrical member rotate relative to each other in one direction, the inner and outer arcuate surfaces thereof are pressed hard against the cylindrical inner surface of the cylindrical member and the cam surface.
The first-mentioned one-way clutch has a problem in that the surface pressure is high when the engaging elements engage the inner and outer rings because the engaging elements are brought into line contact with the inner and outer rings. Thus, when high torque is transmitted between the inner and outer rings, the contact surfaces tend to suffer impressions, peeling and/or cracks. This shortens the life of the clutch.
One way to prevent this problem would be to increase the number of engaging elements used and/or increase their size. But this solution necessarily results in increased size, weight and cost of the entire clutch. Thus, this solution is not always practical or advantageous.
The second-mentioned one-way clutch is free of this problem because the inner and outer arcuate surfaces of the friction members are brought into surface contact, instead of line contact, with the cylindrical inner surface of the cylindrical member and a cam surface. But this clutch has a problem in that the shaft and the cylindrical member cannot be reliably locked together due to a slip that may develop between the inner and outer arcuate surfaces of the friction members and the inner cylindrical surface of the cylindrical member and the cam surfaces. Also, because the inner and outer arcuate surfaces of the friction members have three-dimensional shapes, their dimensions have to be measured using a three-coordinate measuring machine, which is extremely troublesome.
An object of the invention is to provide a one-way clutch which is durable, compact in size, light in weight, less costly, and easy in quality control, by reducing surface pressures when engaged.